1. Field of the Invention
The present invention relates to the field of thin film technology for coating substrates and has particular application to atomic layer deposition.
2. Description of the Related Art
In the field of thin film technology, requirements for thinner deposition layers, better uniformity over increasingly larger area substrates (e.g., ever increasing substrate sizes in flat panel technology as well as the move to 300 mm technology in semiconductor manufacturing), larger production yields, and higher productivity have been, and still are, the driving forces behind emerging technologies developed for coating substrates in the manufacture of various semiconductor devices. Various technologies known in the art exist for applying thin films to substrates, including sputtering or physical vapor deposition (PVD), chemical vapor deposition (CVD) and atomic layer deposition (ALD) methods.
According to the sputtering or PVD method, an inert gas such as argon (Ar), is injected into a vacuum chamber in which high voltage is applied to a target to generate Ar ions in a plasma state. Then, Ar ions are attracted onto the surface of the target such that atoms of the target become dislodged and are deposited onto the surface of the substrate. Although high purity thin films with good adhesion to the substrates can be formed by the sputtering method, it is difficult to ensure uniformity over the entire thin film.
In the CVD method, a desired layer is deposited on a substrate to a desired thickness using decomposition and reaction of gases. First, various gases are injected into a reaction chamber, and chemical reactions between the gases are induced with high energy such as heat, light, or plasma to deposit a thin film having a predetermined thickness on the substrate. The deposition rate of CVD can be increased by controlling either the amount and ratio of the gases or the reaction condition of heat, light, or plasma supplied as the reaction energy. The reactions are so rapid, however, that it is difficult to control the thermodynamic stability of atoms. Another problem is that the close proximity of the reactant and the products of reaction to the deposition surface creates a high probability of the inclusion of reaction products and other contaminants in each deposited layer.
Forming and growing thin films by ALD is based on the principle of the formation of a saturated monolayer of reactive precursor molecules by chemisorption at the deposition surface of the substrate. In ALD, appropriate reactive gaseous precursors are alternately pulsed or injected into a reaction chamber. Each pulse or injection of a reactive precursor is separated by an inert gas purge. The precursor injections react on the surface of the substrate to form a new atomic layer with each full cycle of the process. Thus, atomic layers are built up one over the other in a repetitive process to form a uniform solid layer of film. Each set of process steps performed to form an atomic layer is referred to as a cycle. The repetitive process is repeated for a number of cycles needed to form the desired film thickness.
An essential difference between ALD and other deposition methods is that film growth proceeds stepwise, atomic plane by atomic plane, resulting from a surface reaction between one component element in the gas phase with the other as surface atoms in the growing compound film. As a result, the ALD method achieves improved step coverage and allows for improved control over the thickness and composition of the grown thin film. ALD, although a slower process than CVD, demonstrates a remarkable ability to maintain ultra-uniform thin deposition layers over complex topography.
The unique mechanism of film formation provided by ALD offers several advantages over CVD and other thin film technologies. One advantage is the transparency of ALD to substrate size. For example, a 200 mm substrate will receive a uniform layer equal in thickness to one deposited on a 100 mm substrate processed in the same reactor chamber because of the self-limiting chemisorption phenomena described above. Another distinct advantage of the ALD process is the avoidance of high reactivity of precursors toward one another because chemical species are injected independently into an ALD reactor rather than together. High reactivity, while problematic in some CVD methods, is exploited to an advantage in ALD because the reaction occurs directly on the substrate surface. This high reactivity enables lower reaction temperatures and simplifies process chemistry development. Yet another distinct advantage of ALD is that surface reaction by chemisorption contributes to improved step coverage over complex topography.
A thin film having a high aspect ratio, good uniformity and good electrical and physical properties can be formed by ALD. Moreover, the films deposited by ALD have a lower impurity density than those formed by other deposition methods, and a film thickness of less than about 200 Angstroms with sufficient thickness uniformity that can be obtained with reproducibility. ALD, therefore, is a superior method for achieving uniformity, excellent step coverage, and transparency to substrate size.
While the ALD method can provide a thin film with a high aspect ratio, good uniformity, good electrical, and physical properties, it is problematic in that it has been difficult to create an acceptable commercial process (i.e., one with adequate system throughput) because of the separate deposition of each film layer and the low deposition rate of the process (e.g., typically about 100 Angstroms per minute as compared to about 1,000 Angstroms per minute for CVD). The deposition rate may be increased by increasing the number of deposition cycles per unit time. Thus, there remains a need for improved apparatus and processes for decreasing the overall time to deposit a film by ALD, thereby increasing the overall throughput of products employing layers deposited by ALD.
In certain embodiments, the present invention includes an apparatus for forming a layer on a substrate by atomic layer deposition. In order to commercialize such a process, cyclical steps are performed rapidly due to the large number of atomic layers that generally need to be deposited to achieve the desired thickness of the final product layer. The apparatus of the present invention includes a chamber and a lid that define a reaction chamber therein. A substrate support, such as a heater, supports the substrate in the reaction chamber. The top surface for the substrate support forms a dividing line that, in combination with a virtual shower curtain and walls of the chamber or liner or shadow ring, divides the reaction chamber into a first volume and a second volume or top and bottom volume.
In some embodiments, the virtual shower curtain may be established by providing at least one port through the chamber through which a vacuum may be applied, where upon application of vacuum to the at least one port, a bottom purge of the substrate support is performed and the virtual shower curtain is established between the top surface of the substrate support and the chamber wall, liner or shadow ring, to prevent flow from the top volume to the bottom volume.
In other embodiments, a high conductance showerhead may be mounted to the lid to allow closer positioning of the substrate to the showerhead, compared to use of a standard showerhead. This reduces the volume of the effective reaction chamber, further reducing step and cycle times. A high conductance showerhead described includes at least one distribution hole having a diameter of at least xe2x85x9xe2x80x3. Spacers may also be provided to form a gap between the high conductance showerhead and the lid upon mounting. This gap defines flow pathways for horizontally flowing a reactant and distributing it peripherally about the substrate.
In yet other embodiments, a gas valve mounted on top of the lid of the apparatus may also be designed to reduce step and cycle times. A pair of gas valves may be mounted immediately adjacent the lid to minimize dead volume in flow paths connecting the valves with the showerhead/reaction chamber. The valves may also be mounted flush against the lid, by any variety of techniques or components including welding, threading, or using other attachment methods to attach the valves to the lid or by using screws or other fastening devices to mount the valves on the lid.
In some embodiments, the valves may be ZDV valves, and may be pneumatic valves, solenoid valves or piezo valves. MEMS technology significantly reduces the valve cycle times of the valves used. xe2x80x9cMEMSxe2x80x9d is an acronym used herein to mean xe2x80x9cmicro electro-mechanical systemxe2x80x9d.
In other certain embodiments, an apparatus for forming a layer on a substrate by atomic layer deposition is described herein to include a chamber and a lid which define a reaction chamber therein. A high conductance showerhead is mounted to the lid and has at least one hole having a diameter of at least about xe2x85x9xe2x80x3 for distributing gas therethrough and into the reaction chamber and allowing closer placement of the substrate with regard thereto. A substrate support is provided for supporting the substrate and, together with the formation of a virtual shower curtain, divides the reaction chamber into a first volume and a second volume, the first volume being defined above the top surface of the substrate support and the second volume being below the top surface. At least one port is provided, through which a vacuum may be applied. Upon application of the vacuum, a bottom purge of the substrate support is performed and a virtual shower curtain is established between the top surface of the substrate support and the chamber wall, liner or shadow ring to prevent flow from the first volume to the second volume.
In some embodiments, fast cycle valves may be mounted on top of the lid and fluidly connected with the high conductance showerhead. Each valve may be operated to a valve cycle time of less than about 400 msec. Of course, the fast cycle valves may be operated to a valve cycle time of about 400 msec or more.
In yet other certain embodiments, a method of reducing an amount of time needed to form a film on a substrate by atomic layer deposition is described to include positioning the substrate on a substrate support in a reaction chamber; establishing a virtual shower curtain between the substrate support and walls of the reaction chamber, thereby dividing the reaction chamber into an upper volume above the substrate support and the virtual shower curtain, and a lower volume, below the substrate support and the virtual shower curtain; and distributing a reactant into the upper volume, wherein the virtual shower curtain prevents the reactant from flowing into the lower volume.
In some embodiments, the distributing step may be initiated by turning on a valve connecting a source of the reactant with the reaction chamber, wherein the valve is turned on in less than about 400 msec. Of course, the valve may be turned on in about 400 msec or more.
In other embodiments, the distribution may include flowing the reactant through a showerhead having at least one distribution hole having a diameter of at least xe2x85x9xe2x80x3.